Photoshop brushes do not contain color information. All color is derived from the foreground/background color settings and any brush dynamics, such as the Hue Shift setting.
Brushes are greyscale images in all instances. There is no way to alter that.
You may want to look into smart objects and duplicating those to create multiple instances of colored artwork. This would be the most efficient method. It would allow you to use the same tree over and over but you couldn't simply "brush" the artwork on.
Photoshop CC also has the Tree Filter located under
Filter > Render > Tree. . . which is designed to create trees of various appearance. Exploring that may be of benefit depending upon what you are trying to achieve.
It is possible to get Photoshop to create multiple trees along a path using the
a brush can only contain one color(hue and Saturation), but different alpha(brightness). you can't just put the image to brush preset, instead, you should turn it into black and white image first. and adjust the contrast to what you want(white area will be transparent and black area will be filled with the color when you are using the brush) , and select a high Saturation color when you are using the brush.
If you're on Windows (or can run Windows in a VM) and Paint Shop Pro is an option, the Picture Tubes feature does exactly this. (If it's for a single project, you could conceivably get away with a trial installation, but it's not particularly expensive in any case.) It's not a complete Photoshop replacement by any means, but there are a few things that PSP does that Photoshop doesn't do or doesn't handle nearly as neatly, so it's a good secondary program to have around.